1. Field of the Invention
This invention relates to high frequency transmission line circuit structures, and in particular to a chip having a resident slotline flip-mounted onto a base slotline.
2. Description of Related Art
Mounting flip-chips on a motherboard has been found to be an effective way to connect radio frequency circuit components together. Conducting columns or bumps connect a chip to a motherboard and can be formed using solder, brazing material, plated metal such as gold, epoxy or other conductive adhesive.
Also, in such high frequency application, the use of coplanar transmission lines is well established. Typical examples include coplanar waveguides (ground-signal-ground lines) and slotlines. Slotlines may be formed by spaced opposite-polarity conductors, balanced ground-signal-signal-ground lines, and parallel-strip balanced lines. These coplanar transmission lines are particularly useful because of the simplified structure provided by having the signal-forming conductors on a single plane.
A slotline typically consists of two conductors mounted on a face of a substrate. The conductors are spaced apart to form a slot. The slot is narrow enough for the conductors to be closely coupled, thereby making the conductor edges extending along the slot function as the primary transmission line. Slotlines are typically used to conduct balanced signals, such as are used in push-pull circuits. These circuits may or may not have a corresponding ground plane on the opposite or back side of the substrate. Slotlines also have the advantages of not needing cross-over conductors and having fewer conductors compared to coplanar waveguides.
Slotline conductors are historically made with a semi-infinite width, with each conductor covering all of the portion of the face of the substrate to the side of the slot. A slotline may also be formed by conductor trips having limited widths. Conventionally, a coplanar waveguide consists of two coupled slotlines having the same signal and opposite polarity.
As used herein, a slotline is distinguished from a coplanar waveguide primarily in that the transmission line formed by a slotline for a given signal consists of only two conductors. This is to say, each conductor is only associated with one other conductor to form the transmission line. On the other hand, a coplanar waveguide has three conductors, an inner signal conductor bounded by spaced outer conductors with the outer conductors having the same polarity and sharing in the conduction of current relative to the middle conductor. In a coplanar waveguide, the center conductor is associated with both outer conductors to form the transmission line.
This definition does not preclude the use of other conductors in association with one or both of two slotline conductors for transmitting other signals or multiples of the same signal, for impedance matching or for other functions, so long as the other conductors have arbitrary signal polarities relative to those of the slotline conductors.
Also as used herein, a chip circuit is one or more electrical components formed in or on a chip substrate. Typically, integrated circuits are formed on a chip substrate to form a chip circuit. Other forms of circuit structure may also be formed on a chip. When the chip is flip-mounted onto a base substrate, the chip, containing the chip circuit is referred to as a flip-chip. A flip-chip may contain one or more circuits, elements or devices, each of which may or may not be interconnected on the chip.
In a common amplifier application, the chip circuit is an integrated circuit containing one or more stages of active devices, such as transistors, and other passive devices, such as resistors, capacitors, transmission lines and inductors. In a power chip having a plurality of transistors, the transistors may be individually or collectively connected to the motherboard. Impedance-matching for the composite power transistor can be accomplished on the motherboard substrate in order to minimize the size of the chip substrate. Impedance matching may also be provided on the chip.
As has been mentioned, chips mounted on a base substrate or motherboard may contain simple or complex circuits. More complex circuits are commonly referred to as monolithic microwave integrated circuits, or MMICs for short. When embodied as a microstrip circuit, the chip has a back side ground plane. It is also known to mount MMICs having resident coplanar waveguides by flip mounting the resident coplanar waveguides onto corresponding coplanar waveguides mounted on a base substrate, as is disclosed by Wen et al. in U.S. Pat. No. 5,115,245. The structure disclosed by Wen et al. does not require a back side ground plane and associated vias for connecting circuit components with ground.
Coplanar waveguides however produce a different set of problems. Coplanar waveguides require two ground conductors on opposite sides of a central signal conductor. The additional conductors take up more area of the chip surface. The embodiments disclosed by Wen et al. have a single input or output coplanar waveguide on each end of the chip.
Gaps between conductors of about one mil are required for 50 ohm lines, such as typical coplanar waveguides. This is a severe manufacturing problem because of the difficulty of producing gaps consistently with such small spacing. This condition is exacerbated by the characteristic of low impedance interconnects to be more sensitive to minute variations in manufacture than high impedance interconnects.
Coplanar waveguides also exhibit several radiation modes that dissipate energy and produce coupling between circuits. Cross coupling can cause oscillation and produce pass band ripples or notches. Coplanar waveguides are also inherently difficult to launch directly to a shielded waveguide.